Rename the first "about codabench" block

Just a post it to remember to rename this block:

Change Github link from https://github.com/codalab/competitions-v2 to https://github.com/codalab/codabench

Make Popular Benchmarks and Featured Benchmarks links to public benchmarks:

Todos

• Rename About Codabench to Get started
• Update codabench github link and tutorial link
• Make Popular Benchmarks and Featured Benchmarks links that redirect to the competition lists (https://www.codabench.org/competitions/public/?page=1)